Ten years ago when United
Airlines Flight 811 took off from Honolulu for Sydney, two thin wires rubbed
together and an electrical short circuit powered open the latches on the
cargo door in the belly of the Boeing 747. When the aircraft reached
23,000 feet the huge door burst open and ripped off in the slipstream,
tearing away cabin structure. Nine passengers were blown out through
the hole, their bodies were never recovered. So began the saga of aging aircraft
wire.

Since then wiring faults may have killed hundreds more in the
crash of TWA Flight 800 in July 1996 and Swissair Flight 111 in September
1998. Every airliner has hundreds of kilometers
of wire and more is being added to support sophisticated passenger entertainment
systems. All wire deteriorates to some degree and much of it is not
accessible for inspection. Slowly, ever so slowly the aviation industry
is acknowledging that a hazard exists. Finding remedies will take
much longer.

The cause of the United Airlines
accident never attracted the attention it deserved partly because blame
was initially directed at the cargo handler. He was accused of not
closing the door properly; just another human error.

It took eighteen months to find the
door and recover it from 5 km down on the ocean floor. The technology
to do this was the same as that recently used to find the Kennedy aircraft;
trajectory analysis from radar records, assessment of ocean currents, sonar
search, unmanned and manned submersible vehicles.

After studying the door and its internal
wiring the National Transportation Safety Board changed its earlier findings. The probable cause became “…a faulty switch or wiring in the door control
system which permitted electrical actuation of the door latches toward
the unlatched position after initial door closure and before takeoff.
Contributing to the cause of the accident was a deficiency in the design
of the cargo door locking mechanisms, which made them susceptible to deformation,
allowing the door to become unlatched after being properly latched and
locked.” (followup)

Physical clues from the accident
were supported by a later incident when a cargo door opened uncommanded
on a plane about to board passengers. Again the insulation on some
wires was abraded and burned where the bundle going into the door flexed
every time the door opened.

Another reason why wire defects attracted
little attention at that time was because the industry was preoccupied
with fixing problems of aging aircraft structures. A year before
the United Airlines accident, an old Aloha Airlines Boeing 737 lost a large
portion of its fuselage during a short inter-island flight. Failure
of riveted skin joints exposed dangers of fatigue, corrosion, faulty repairs
and undue reliance on inspections, instead of fixing known defects.

First Officer Madeline Tompkins was
flying the Aloha plane when it burst apart. She told investigators
how her head was jerked back by the blast and the Captain told how he looked
back into the cabin and saw blue sky where the roof had been. Six
metres of cabin structure had disappeared; 89 passengers were strapped
in but being battered by the slipstream and speared with debris.
Some had seen flight attendant Clarabelle Lansing blasted from the plane.
Miraculously the plane landed safely but most passengers were injured.
(Read Story)

Reasons for the fuselage failure
were soon determined, none were unexpected, and some had been known for
twenty years. In particular fuselage assembly was faulty on early
737’s. Airlines had to strengthen the joints or repeatedly inspect
them; few did the modification and many became complacent about the inspections.

Disagreement over the risks had gone
on for years and just two weeks before the accident Boeing rejected warnings
from the Australian regulators. Afterwards it took a great effort
by the US Congress, enlightened airlines and a few regulators to convince
the industry that the underlying problems were endemic and not limited
to Boeing or to the 737.

A memorable document from the US
Federal Aviation Administration rejected an (Australian) assertion that
failures on the 737 could equally happen on the Boeing 727. FAA said
there was no evidence. The very next sentence admitted that a similar
defect had just occurred on the 727 so action was needed. Obviously
the first sentence was written before the incident and should have been
deleted before the document got published!

Eventually aging structure problems
were acknowledged and addressed by the whole industry. Every structural
defect was reassessed and rectified; inspection alone was no longer allowed.
Corrosion prevention and control programs were instituted and maintenance
was intensified.

Fixing aging structures has cost
the industry billions and billions of dollars. But the cost per ticket
is less than $5, and well worthwhile.

Meanwhile the hazards of aging wire
were disregarded until TWA 800 exploded off the coast of Long Island. Flammable
vapor in a near empty fuel tank ignited. Why is not known,
but low voltage wires that go to quantity sensors in the tank might have
shorted to high voltage wires elsewhere in the plane.

Again action was forced at a political
level. The White House Commission on Aviation Safety and Security
insisted that “in cooperation with airlines and manufacturers, the FAA’s
Aging Aircraft program should be expanded to cover non-structural systems”.

Belatedly spot checks of maintenance
on old aircraft found that the aging structures program overlooked corrosion
of control systems and wiring too was deteriorating with age. Also
alarmingly, wiring bundles were being contaminated by metal shavings; usually
debris from fixing the structural problems!

So research began into better detection
of faulty wires and technical interchange started between FAA, Defense
and NASA on wire deterioration. Meanwhile evidence emerged that a
cockpit fire that caused the crash of Swissair 111 may have been due to
defective insulation on aging wires.

The suspect insulation material is
caused KAPTON. It is light and flexible when new but embrittles and
cracks over time. Also when wire short circuit their outer
sheaf chars and becomes highly conductive. Thus bundles of wire can
quickly burn along their length. For these reasons KAPTON has been
banned from most military aircraft and is avoided in new civil aircraft.
Nevertheless KAPTON is used on about 65 % of airliners in service.

KAPTON was not implicated in the
United 811 accident or TWA 800. Both of these had older types of
wire that too deteriorated.

Aging wire concerns are supposed
to be addressed jointly by manufacturers, airlines and regulators.
The first two may not be keen to probe deeply because of horrendous cost
implications if something is seriously wrong. And regulators around
the world are being downsized, outsourced and stripped of technical expertise.
The US Government watchdog, the General Accounting Office recently found
that FAA’s inspection initiatives were a shambles.

Do not hope for pro-active intervention
by our Civil Aviation Safety Authority. Its policy now is to slavishly
mimic the FAA. Though CASA sneaks in a few rules that are more lax
than elsewhere.

Something else has changed.
Whereas professionals in private resolved aging structures, aging wire
is getting the full glare of the Internet. A site sponsored by the
widow of a Swissair victim,
provides more technical data than any person can digest. She says
she is spending her “blood money” pay-out to ensure informed public debate.

Now everyone can learn the risks
of aging wire in aircraft, but whether the risks can be managed and reduced
remains to be seen.

By Martin B Aubury
Fellow of the Royal Aeronautical
Society
Former Head of Aircraft Structures
at (Australian) Civil Aviation Authority